The present invention relates to the detection of ice accretion on the surface of an object which, during use, is subjected to atmospheric conditions in which an ice layer may form.
The reliable detection of ice accretion is important in several applications. One application of particular significance is in the all-weather operation of aircraft. During such operation, ice accretion on critical flight components must be detected with sufficient sensitivity that an operator can receive a "Caution and Warning" alert before a dangerously thick ice layer can form.
The above considerations come into play even more forcefully for moving-wing aircraft such as helicopters. Helicopter exposure to icing conditions has increased over the past decade because of the increase in routine instrument flight operations. Helicopter rotor icing can present a significant hazard due to increased torque requirements, aerodynamic performance degradation, control problems, and severe vibrations resulting from uneven ice shedding.
Because rotating components often experience icing conditions that are significantly different from those experienced by the fuselage, visual evidence of ice accretion on non-rotating structures is not adequate for Caution and Warning applications. Moreover, indirect indications of icing, such as torque rise or vibration increase, typically emerge only after a hazardous condition exists. On the other hand, the direct measurement of ice accretion on the rotating components would provide the earliest detection and most accurate measurement of a potential icing hazard.
Ice detection is also important in the management of ice protection systems. A common type of ice protection system is the electrothermal de-icer, which requires so much electrical power that airfoils are typically de-iced in segments. Without effective ice detection, it is necessary to overheat and continually cycle the electrothermal de-icing system to assure adequate ice protection. If an effective ice detector were available, electrothermal de-icers could be operated more efficiently and effectively.
The ice detectors most commonly employed for aviation applications are the direct-contact-type devices. In the typical direct-contact detector, a probe or surface-mounted transducer senses the presence of ice through a variety of physical mechanisms, such as by measuring resonant frequency shifting, optical blocking, heat capacity, electrical capacity or ultrasonic thickness. Direct-contact detectors are described in A. Heinrich et al., "Aircraft Icing Handbook--Vol. I", U.S. Dept. of Transportation Publication No. DOT/FAA/CT - 88/8-1 (March 1988).
Contact-type ice detectors have not been successfully used for direct measurement of rotor ice accretion due to the severe difficulties encountered in instrumenting a rotor. These difficulties include mounting restrictions imposed by structural limitations of the rotor blade, transmission of information between the rotating and non-rotating frames, acceleration limitations of the sensors, and erosion problems. As a result, a contact-type ice detector is typically mounted on the fuselage and the rotor's icing condition is inferred from the fuselage's condition. Such indirect ice detection has had only limited success because of the complex dependence of the icing process on the temperature, liquid water content, and velocity, all of which vary significantly between the fuselage and the rotor, as well as along and across each rotor blade.
Direct ice detection via remote sensing offers significant advantages for monitoring rotating blades. If ice accretion can be remotely detected by sensors mounted in the fuselage, then a direct measurement of rotor ice accretion can be accomplished without the difficulties identified above. Several techniques, including high-speed video and active microwave systems, have been investigated for remote ice detection.